Two basic methods of measuring differential impedances between two sensor elements are presently in use. The first method utilizes a DC bridge configuration with either an instrumentation amplifier or a gain amplifier stage followed by a bias stage type of amplifier. Even though amplifier technology has improved significantly, temperature drift is a problem with either of the foregoing circuit configurations. The signal level of the differential sensor is low enough that significant shifts with temperature occur. Temperature compensation is possible, but requires significantly more manufacturing and testing time which increases overall cost. In addition, such temperature compensation requires more electronic components in the analog portion of the circuitry and more memory capacity in the digital portion of the circuitry.
The second method of measuring differential impedances between two sensor elements utilizes an AC bridge configuration and requires a sine wave generator circuit to provide an input to the common leg between the sensor elements. The active and reference legs of the sensor elements are connected to the primary side of a signal transformer having a center tap connected to common. The difference between the current in the active leg minus the current in the reference leg is present at the secondary winding of the transformer. This differential signal is then amplified through a two-stage active low pass filter and then rectified by an AC to DC converter. An inverter comparator circuit is necessary to phase the AC to DC converter stage. In addition, an active low pass filter is required to further filter the ripple existing in the signal as it exits from the AC to DC converter. This second method of measuring differential impedances improves the temperature characteristics of the resulting circuitry but requires additional electronic components in the form of an inverter comparator circuit and an active low pass filter. These additional circuit components significantly increase the overall costs of the resulting circuitry.
Because of the foregoing limitations with respect to the prior art approaches for measuring differential impedances, it has become desirable to develop circuitry for measuring differential impedances that is not effected by temperature changes and requires a minimum number of circuit components.